I'm running 18" wheels with 235/55/18 tires. Just wondering if I'm supposed to use the same stock tire pressure of 32/34psi or something else. I did notice the spare, which is a different size tire, uses a significantly higher pressure, so I'm thinking there's probably a different optimum pressure for lower profile, wider tires?

Thanks for any info.

SRLNCLT

04-26-2005, 01:37 PM

Go with whatever it says on the tire.
L.

paulj

04-26-2005, 01:48 PM

Most, if not all, tires note a maximum pressure, typically 44 psi. The spec tables also a list load rating and pressure pair. But you will note that the pressure that Honda recommends 32/34 is less than this (even for the stock tires). Honda's number takes into account the weight of the car.

I have not seen a good discussion about how to adjust pressures when changing tire size. A starting point is the pressure at which the load is spread out evenly across the width of the tread. Higher pressure shifts lifts the edges of the tread off the ground and shifts wear to the center. Lower pressure puts more wear on the edges, and lets tire flex more (and heat up in the process).

paulj

Ranger

04-26-2005, 03:56 PM

The higher pressure recommended for the spare is not due to size difference, it is due to construction differences in the tires.

paulj

04-26-2005, 04:51 PM

I would disagree about the spare. It is constructed to take a higher pressure, but this so it can support the desired load on a smaller contact patch. The spare is 2/3 the width of the stock tires. Assuming that the contact patch length is the same, the spare's contact patch area will be 2/3 of the stock one, so it should have 3/2 the pressure. At 60psi, it is harder than that, so I'd expect the length of its patch to be less. Patch area is likely to be half of the stock area.

On one of the tire size spreadsheets floating around, I added tire load data (from typical spec tables) and a tire cross section area calculation (section_width x (profile x section_width)). For stock tires this is 215mm x 215mmx.7 = 32,000 mm^2. With a load rating of 1700 lb, load_rating / cross_section = .05 lb/mm^2. For a range of tire sizes, this ratio is in the 0.048 to 0.058 range. Except for the spare, which is more like 0.12 (about double).

Roughly speaking I am thinking of the weight of the car being supported on 'slab' of air directly under the metal wheel.

It might be valid to argue that the 'reference' pressure for a given car and tire, should vary with this tire cross_section, or equivalently with the load rating. 215/70/16 and 235/60/16 have nearly the same load rating, so should run at about the same pressure. 225/60/16 has a lower rating, so should be run at a higher pressure. 235/70/16 could be run lower. A lighter car can run at lower pressures for a given tire size (e.g. RAV4 on the same size tires as the Element calls for 28/26 psi).

In practice there may be reasons for running the tires harder than this 'reference' pressure. One reason for going wider and lower profile is to improve handling. Higher pressure also improves handling, since it keeps the tire from deforming as much. But too high pressure reduces contact patch, reducing grip.

I found that running my Cooper AT tires a bit on the hard side (about 4 psi) reduces road noise, and gives the steering a lighter feel. This is because it lifts the edges of the tire off the ground a bit. With this particular design the tread in the shoulder is more aggressive than in the center. However for better traction and ride on rough roads I should lower the pressure. 4x4 drivers often drop down to 24 psi, or even 15psi in soft stand, seeking to increase traction and floatation. These pressures are too low for high speed highway driving.